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Applying paint coats

Q: In spraying from a can (Scale Auto Feb. 2002), Pat Covert suggests waiting about 20 minutes between the initial mist coats to let the paint set up. When applying the heavier wet coats, he doesn't mention any wait. Should these heavier coats be allowed to set up, or can they be applied immediately after one another? In the same article, Pat advises waiting seven to ten days for an enamel paint job to cure before using a polishing kit. In a recent SA article, George Bojaciuk recommends giving enamel paint at least 30 days to "gas out" before polishing. I'm a little confused, but grateful for any help.

- Jim Daly
Norwich, Conn.

KEN: The answer to your question, Jim, is about as exciting as watching paint dry - literally.

The solvents and propellants used to transfer pigment to the object being painted begin to evaporate or "volatize" as the paint reaches your model. Simply put, when all the vapors have completely volatized, the paint is dry. This volatilization (also known as "gassing out") occurs from the surface of the paint, but there's a catch: paint dries from the top - the layer exposed to the most air - forming a skin that makes it more difficult for the trapped vapors to escape. Paint generally sets up or forms a skin quickly, then takes a while to completely "gas out" beneath the surface.

When thin coats are applied in rapid succession, the layers' surfaces can set up enough to keep the colors from running together, but still gas out as a single coat. On heavier coats, when the surface skin gets too hard, a second coat will only seal the first coat and not allow any of the vapors to escape through the skin. You run the risk that over time, those solvents will turn around and try to get out through the plastic car body if the primer isn't strong enough to hold them back. That's why labels on some rattle cans say "apply a second coat within 20 minutes or after 24 hours."

Personal preference, the type of paint, and the number of coats come into play when it comes to polishing out your model, which is why Pat and George may recommend different drying times. Relative humidity in your area is also an important factor in drying time.

When in doubt, polish a small, inconspicuous section of the model. If the surface won't polish to a brilliant sheen, wait a few more days and try it again.


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